Heat sink structure with a vapor-permeable membrane for two-phase cooling

ABSTRACT

A heat sink, and cooled electronic structure and cooled electronics apparatus utilizing the heat sink are provided. The heat sink is fabricated of a thermally conductive structure which includes one or more coolant-carrying channels coupled to facilitate the flow of coolant through the coolant-carrying channel(s). The heat sink further includes a membrane associated with the coolant-carrying channel(s). The membrane includes at least one vapor-permeable region, which overlies a portion of the coolant-carrying channel(s) and facilitates removal of vapor from the coolant-carrying channel(s), and at least one orifice coupled to inject coolant onto at least one surface of the coolant-carrying channel(s) intermediate opposite ends of the channel(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/676,242, entitled “Heat Sink Structure with a Vapor-PermeableMembrane for Two-Phase Cooling,” filed Nov. 14, 2012, which publishedMar. 28, 2013, as U.S. Patent Publication No. 2013/0077246 A1. U.S.application Ser. No. 13/676,242 is a continuation of U.S. applicationSer. No. 13/189,597, entitled “Heat Sink Structure with aVapor-Permeable Membrane for Two-Phase Cooling,” filed Jul. 25, 2011,and which published Jan. 31, 2013, as U.S. Patent Publication No.2013/0027878 A1. Each of these applications is hereby incorporatedherein by reference in its entirety.

BACKGROUND

The power dissipation of integrated circuit chips, and the modulescontaining the chips, continues to increase in order to achieveincreases in processor performance. This trend poses a cooling challengeat both the module and system level. Increased airflow rates are neededto effectively cool high power modules and to limit the temperature ofthe air that is exhausted into the computer center.

In many large server applications, processors along with theirassociated electronics (e.g., memory, disk drives, power supplies, etc.)are packaged in removable node configurations stacked within a rack orframe. In other cases, the electronics may be in fixed locations withinthe rack or frame. Typically, the components are cooled by air moving inparallel airflow paths, usually front-to-back, impelled by one or moreair moving devices (e.g., fans or blowers). In some cases it may bepossible to handle increased power dissipation within a single node byproviding greater airflow, through the use of a more powerful air movingdevice or by increasing the rotational speed (i.e., RPMs) of an existingair moving device. However, this approach is becoming problematic at therack level in the context of a computer installation (i.e., datacenter).

The sensible heat load carried by the air exiting the rack is stressingthe ability of the room air-conditioning to effectively handle the load.This is especially true for large installations with “server farms” orlarge banks of computer racks close together. In such installations,liquid cooling (e.g., water cooling) is an attractive technology tomanage the higher heat fluxes. The liquid absorbs the heat dissipated bythe components/modules in an efficient manner. Typically, the heat isultimately transferred from the liquid to an outside environment,whether air or other liquid coolant.

BRIEF SUMMARY

In one aspect, a method of facilitating extraction of heat from aheat-generating electronic component is provided. The method includes:providing a heat sink comprising a thermally conductive structureincluding at least one coolant-carrying channel, and a membranestructure associated with the at least one coolant-carrying channel, themembrane structure comprising at least one vapor-permeable region, atleast a portion of the at least one vapor-permeable region overlying aportion of the at least one coolant-carrying channel and facilitatingremoval of vapor from the at least one coolant-carrying channel, whereinthe membrane structure further includes at least one orifice coupled toinject coolant onto at least one surface of the at least onecoolant-carrying channel intermediate ends of the at least onecoolant-carrying channel. In addition, the heat sink includes: at leastone vapor transport channel in fluid communication with the at least onevapor-permeable region of the membrane structure, the at least one vaportransport channel facilitating exhausting of vapor egressing across theat least one vapor-permeable region of the membrane structure from theat least one coolant-carrying channel; at least one coolant exhaustchannel in fluid communication with the at least one coolant-carryingchannel and facilitating exhausting of coolant from the at least onecoolant-carrying channel; and a coolant exhaust outlet port coupled influid communication with the at least one coolant exhaust channel, and avapor outlet port coupled in fluid communication with the at least onevapor transport channel for separately discharging coolant exhaust fromthe at least one coolant exhaust channel and vapor from the at least onevapor transport channel. The method further includes coupling the heatsink to the at least one heat-generating electronic component so thatheat generated by the at least one heat-generating electronic componentis dissipated to coolant within the at least one coolant-carryingchannel of the heat sink, wherein vapor generated within the at leastone coolant-carrying channel can exhaust from the at least onecoolant-carrying channel across the at least one vapor-permeable regionof the membrane.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered part of the claimedinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is an elevational view of one embodiment of a cooled electronicsrack comprising one or more heat-generating electronic components, andemploying one or more heat sinks, in accordance with one or more aspectsof the present invention;

FIG. 2 is a schematic of one embodiment of an electronic subsystem ornode of an electronics rack, wherein an electronic component andassociated heat sink are cooled by system coolant provided by one ormore modular cooling units disposed within the electronics rack, inaccordance with one or more aspects of the present invention;

FIG. 3 is a schematic of one embodiment of a modular cooling unit for acooled electronics rack such as depicted in FIGS. 1 & 2, in accordancewith one or more aspects of the present invention;

FIG. 4 is a plan view of one embodiment of an electronic subsystemlayout illustrating multiple heat sinks cooling multiple electroniccomponents of the electronic subsystem, in accordance with one or moreaspects of the present invention;

FIG. 5A is a cross-sectional elevational view of one embodiment of acooled electronic structure comprising a heat-generating electroniccomponent and a heat sink with a vapor-permeable membrane, and takenalong lines 5A-5A in FIGS. 5B & 5D, in accordance with one or moreaspects of the present invention;

FIG. 5B is a cross-sectional plan view of the cooled electronicstructure of FIG. 5A, taken along line 5B-5B thereof, in accordance withone or more aspects of the present invention;

FIG. 5C is a cross-sectional plan view of the cooled electronicstructure of FIG. 5A, taken along line 5C-5C thereof, in accordance withone or more aspects of the present invention;

FIG. 5D is a cross-sectional plan view of the cooled electronicstructure of FIG. 5A, taken along line 5D-5D thereof, in accordance withone or more aspects of the present invention;

FIG. 6 is a schematic one embodiment of a cooled electronics apparatuscomprising an electronic subsystem or node with multiple heat sinks andillustrating rack-level vapor separation and condensing, in accordancewith one or more aspects of the present invention;

FIG. 7 depicts a more detailed embodiment of the cooled electronicsapparatus of FIG. 6, illustrating rack-level vapor separation andcondensing, in accordance with one or more aspects of the presentinvention;

FIG. 8A is a cross-sectional elevational view of another embodiment of acooled electronic structure, and taken along lines 8A-8A in FIGS. 8B &8D, in accordance with one or more aspects of the present invention;

FIG. 8B is a cross-sectional plan view of the cooled electronicstructure of FIG. 8A, taken along line 8B-8B thereof, in accordance withone or more aspects of the present invention;

FIG. 8C is a cross-sectional plan view of the cooled electronicstructure of FIG. 8A, taken along the line 8C-8C thereof, in accordancewith one or more aspects of the present invention;

FIG. 8D is a cross-section plan view of the cooled electronic structureof FIG. 8A, taken along line 8D-8D thereof, in accordance with one ormore aspects of the present invention;

FIG. 9 is a schematic of one embodiment of a cooled electronicsapparatus employing node-level merging of vapor and coolant exhaust, andrack-level vapor separation and condensing, in accordance with one ormore aspects of the present invention;

FIG. 10 is a schematic view of an alternate embodiment of a cooledelectronics apparatus comprising multiple cooled electronic structuresand rack-level merging of vapor and coolant exhaust, as well asrack-level vapor separation and condensing, in accordance with one ormore aspects of the present invention;

FIG. 11 is a more detailed schematic view of the cooled electronicsapparatus of FIG. 10, in accordance with one or more aspects of thepresent invention;

FIG. 12A is a cross-sectional elevational view of another embodiment ofa cooled electronic structure comprising a heat-generating electroniccomponent and a heat sink with a vapor-permeable membrane, and takenalong line 12A-12A in FIG. 12B, in accordance with one or more aspectsof the present invention;

FIG. 12B is a cross-sectional plan view of the cooled electronicstructure of FIG. 12A, taken along line 12B-12B thereof, in accordancewith one or more aspects of the present invention; and

FIG. 12C is a cross-sectional plan view of the cooled electronicstructure of FIG. 12A, taken along line 12C-12C thereof, in accordancewith one or more aspects of the present invention.

DETAILED DESCRIPTION

As used herein, the terms “electronics rack” and “rack unit” are usedinterchangeably, and unless otherwise specified include any housing,frame, rack, compartment, blade server system, etc., having one or moreheat generating components of a computer system or electronics system,and may be, for example, a stand alone computer processor having high,mid or low end processing capability. In one embodiment, an electronicsrack may comprise a portion of an electronic system, a single electronicsystem or multiple electronic systems, for example, in one or moresub-housings, blades, books, drawers, nodes, compartments, etc., havingone or more heat-generating electronic components disposed therein. Anelectronic system(s) within an electronics rack may be movable or fixedrelative to the electronics rack, with rack-mounted electronic drawersand blades of a blade center system being two examples of electronicsystems (or subsystems) of an electronics rack to be cooled.

“Electronic component” refers to any heat generating electroniccomponent of, for example, a computer system or other electronic systemrequiring cooling. By way of example, an electronic component maycomprise one or more integrated circuit dies, chips, modules and/orother heat-generating electronic devices to be cooled, such as one ormore processors, memory modules and/or memory support structures.Further, as used herein, the terms “heat sink” and “coolant cooled heatsink” refer to thermally conductive structures having one or morechannels (or passageways) form therein or passing therethrough, whichfacilitate the flow of coolant through the structure. One example, thecoolant carrying channels comprise microchannels having a hydraulicdiameter of 1.0 mm or less, for example, in the range of approximately0.1 mm to 0.5 mm.

As used herein, “liquid-to-liquid heat exchanger” may comprise, forexample, two or more coolant flow paths, formed of thermally conductivetubings (such as copper or other tubing) in thermal or mechanicalcontact with each other. Size, configuration and construction of theliquid-to-liquid heat exchanger can vary without departing from thescope of the invention disclosed herein. Further, “data center” refersto a computer installation containing one or more electronics racks tobe cooled. As a specific example, a data center may include one or morerows of rack-mounted computing units, such as server units.

One example of facility coolant and system coolant is water. However,the concepts disclosed herein are readily adapted to use with othertypes of coolant on the facility side and/or on the system side. Forexample, one or more of the coolants may comprise a brine, a dielectricliquid, a fluorocarbon liquid, a liquid metal, or other similar coolant,or refrigerant, while still maintaining the advantages and uniquefeatures of the present invention.

Reference is made below to the drawings (which are not drawn to scalefor ease of understanding), wherein the same reference numbers usedthroughout different figures designate the same or similar components.

FIG. 1 depicts one embodiment of a liquid-cooled electronics rack 100which employs a liquid-based cooling system. In one embodiment,liquid-cooled electronics rack 100 comprises a plurality of electronicsubsystems or nodes 110, which may comprise processor or server nodes,as well as a disk enclosure structure 111. In this example, a bulk powerassembly 120 is disposed at an upper portion of liquid-cooledelectronics rack 100, and two modular cooling units (MCUs) 130 aredisposed in a lower portion of the liquid-cooled electronics rack. Inthe embodiments described herein, the coolant is assumed to be water oran aqueous-based solution, again, by way of example only.

In addition to MCUs 130, the cooling system includes a system watersupply manifold 131, a system water return manifold 132, andmanifold-to-node fluid connect hoses 133 coupling system water supplymanifold 131 to electronics structures 110, 111 and node-to-manifoldfluid connect hoses 134 coupling the individual electronics subsystems110, 111 to system water return manifold 132. Each MCU 130 is in fluidcommunication with system water supply manifold 131 via a respectivesystem water supply hose 135, and each MCU 130 is in fluid communicationwith system water return manifold 132 via a respective system waterreturn hose 136.

As illustrated, heat load of the electronic structures is transferredfrom the system water to cooler facility water supplied by facilitywater supply line 140 and facility water return line 141 disposed, inthe illustrated embodiment, in the space between a raised floor 145 anda base floor 165.

FIG. 2 schematically illustrates operation of the cooling system of FIG.1, wherein a liquid-cooled heat sink 200 is shown coupled to anelectronic component 201 of an electronic subsystem 110 within theelectronics rack 100. Heat is removed from electronic component 201 viathe system coolant circulated via pump 220 through heat sink 200 withinthe system coolant loop defined by liquid-to-liquid heat exchanger 221of modular cooling unit 130, lines 222, 223 and heat sink 200. Thesystem coolant loop and modular cooling unit are designed to providecoolant of a controlled temperature and pressure, as well as controlledchemistry and cleanliness to the electronic component(s). Furthermore,the system coolant is physically separate from the less controlledfacility coolant in lines 140, 141, to which heat is ultimatelytransferred.

FIG. 3 depicts a more detailed embodiment of a modular cooling unit 130,in accordance with an aspect of the present invention. As shown in FIG.3, modular cooling unit 130 includes a facility coolant loop whereinbuilding chilled, facility coolant is supplied 310 and passes through acontrol valve 320 driven by a motor 325. Valve 320 determines an amountof facility coolant to be passed through liquid-to-liquid heat exchanger221, with a portion of the facility coolant possibly being returneddirectly via a bypass orifice 335. The modular cooling unit furtherincludes a system coolant loop with a reservoir tank 340 from whichsystem coolant is pumped, either by pump 350 or pump 351, into the heatexchanger 221 for conditioning and output thereof, as cooled systemcoolant to the associated rack unit to be cooled. The cooled systemcoolant is supplied to the system supply manifold and system returnmanifold of the liquid-cooled electronics rack via the system watersupply hose 135 and system water return hose 136.

FIG. 4 depicts one embodiment of an electronic subsystem 110 layoutwherein one or more air moving devices 411 provide forced air flow 415to cool multiple devices 412 within electronic subsystem 110. Cool airis taken in through a front 431 and exhausted out a back 433 of thedrawer. The multiple devices to be cooled include multiple processormodules to which coolant-cooled heat sinks 420 (of a cooling system) arecoupled, as well as multiple arrays of memory modules 430 (e.g., dualin-line memory modules (DIMMs)) and multiple rows of memory supportmodules 432 (e.g., DIMM control modules) to which air-cooled heat sinksare coupled. In the embodiment illustrated, memory modules 430 and thememory support modules 432 are partially arrayed near front 431 ofelectronic subsystem 110, and partially arrayed near back 433 ofelectronic subsystem 110. Also, in the embodiment of FIG. 4, memorymodules 430 and memory support modules 432 are cooled by air flow 415across the electronic subsystem.

The illustrated liquid-based cooling system further includes multiplecoolant-carrying tubes connected to and in fluid communication withcoolant-cooled heat sinks 420. The coolant-carrying tubes comprise setsof coolant-carrying tubes, with each set including (for example) acoolant supply tube 440, a bridge tube 441 and a coolant return tube442. In this example, each set of tubes provides liquid coolant to aseries-connected pair of heat sinks 420 (coupled to a pair of processormodules). Coolant flows into a first heat sink of each pair via thecoolant supply tube 440 and from the first heat sink to a second heatsink of the pair via bridge tube or line 441, which may or may not bethermally conductive. From the second heat sink of the pair, coolant isreturned through the respective coolant return tube 442. In an alternateimplementation, tubing is provided for separately passing coolant inparallel through the heat sinks of the electronic subsystem.

In one embodiment, the above-described cooling system can be employedwith single-phase liquid-cooling. However, such a system requires alarge liquid flow rate, and correspondingly large, high-power pumps, toavoid the liquid boiling and minimize sensible heating of the fluid asit absorbs the heat dissipated. The flow rate and pump power requiredmay be reduced by an order of magnitude by leveraging the large, latentheat of vaporization, allowing the liquid to boil. Flow boiling enjoyshigh heat transfer coefficients, which can facilitate reducing thejunction-to-ambient module thermal resistance, and can couple the moduletemperature to the boiling point of the coolant (or working fluid),resulting in better temperature uniformity.

However, flow boiling in the confined flow geometries of small heat sinkchannels, and small impingement jets in the heat sink, result in adetrimental rise in pressure due to bubble nucleation, bubble growth andadvection of the vapor phase. The rise in pressure shifts saturationconditions, delaying the onset of boiling, and also results in thedevelopment of flow instabilities and flow maldistribution at the heatsink and node level, which can lead to premature liquid dryout. Theseissues have made flow boiling microstructures difficult to implement.

As used herein, a “microchannel”, “micro-jet” or “microstructure” refersto a structure having a characteristic dimension less than 1.0 mm, forexample, of approximately 0.5 mm or less. In one implementation, themicrochannel has a hydraulic diameter of approximately 100 microns, andthe jet channel (or jet orifice) has a diameter less than 100 microns.In the implementations described herein, the jet orifice diameter isassumed to be less than the microchannel width, since the jet orificeinjects coolant into the microchannel(s) of the heat sink.

Disclosed hereinbelow are various heat sink structures which combinelocal jet impingement of coolant (through jet nozzles (or jet orifices))with local vapor removal via a porous, vapor-permeable membrane, whichminimizes the various challenges encountered during flow boiling inmicrostructures. The microchannels provide a larger heat transfer areaand improved thermal performance as compared to larger, conventionalchannels, and by incorporating a vapor-permeable membrane within theheat sink structure, vapor generated within the microchannels can escapethe confined microchannel geometry directly into a separate vaportransport channel/plenum. This local removal of vapor provides severaladvantages, including: a reduced two-phase flow pressure drop and areduced required pumping power for circulating coolant through the heatsink structure(s); a lower and more uniform coolant saturationtemperature within the heat sink structure; an improved heat transfercoefficient and reduced heat sink thermal resistance due to phasechange; improved wetting and improved jet impingement; and a reducedpossibility of flow instabilities which might lead to premature dryoutwithin the heat sink.

The separated vapor can be reintroduced into the coolant exhaust fromthe cooling microchannels within the heat sink structure itself, or at anode level within an electronics rack comprising the heat sinkstructure. Alternatively, the vapor may be piped directly to arack-level manifold, as explained further below. Secondary,buoyancy-driven vapor separation occurs in the rack manifold, with vaporrising to a condenser disposed in the upper portion of the rack unit.The vapor is then condensed back to liquid, which rejoins the liquidcoolant returning to the modular cooling unit, where the liquid can becooled and pumped back to the nodes of the electronics rack. In oneembodiment, the coolant flowing through the heat sink structurescomprises water, and the membrane is a porous, hydrophobic membrane.Further, in one embodiment, the membrane may be modified to have aspatially-varying porosity and stiffness, which allows for both theinjection of fluid, through jet orifices provided in rigid portions ofthe membrane, and local removal of vapor generated within themicrochannels. Alternatively, a plate mask could be associated with thevapor-permeable region of the membrane to define a multilayer structure,which comprises one or more coolant injection regions and one or morevapor removal regions from the microchannels. Note that in theembodiments described herein, the membrane, or the membrane and platemask structure, overlie and form part of the coolant-carrying channelsso as to be exposed to vapor within the coolant-carrying channels of theheat sink. For example, in one embodiment, the membrane forms a topportion of each of the coolant-carrying channels of the heat sink.

FIGS. 5A-5D depict one embodiment of a cooled electronic structure,generally denoted 500, in accordance with one or more aspects of thepresent invention. Cooled electronic structure 500 includes, in thisembodiment, an electronic component 510, such as an electronic module,mounted to a printed circuit board 501 with an associated back plate 502(for example, a metal back plate). A heat sink 520 is mechanicallycoupled via securing mechanisms 505 to back plate 502 of printed circuitboard 501, which provide a compressive load forcing heat sink 520 ingood thermal contact with electronic component 510. Electronic component510 includes, in this embodiment, an integrated circuit chip 511connected to a chip carrier or substrate 513 via, for example, a firstplurality of solder ball connections 512. Similarly, substrate 513 iselectrically connected to printed circuit board 501 via, for example, asecond plurality of solder ball connections 514. A thermally conductivecap 516 is interfaced to integrated circuit chip 511 via a first thermalinterface material 515, such as a silicone-based paste, grease, or pad,or epoxy or solder. A second thermal interface material 517 facilitatesthermal interfacing of cap 516 to heat sink 520.

In this embodiment, heat sink 520 comprises a multilayer heat sink witha heat sink base 521, a membrane structure 523 and a heat sink cap 526,which are respectively depicted in cross-sectional plan view in FIGS.5B-5D. Unless otherwise indicated, referring collectively to FIGS.5A-5D, heat sink base 521 comprises one or more coolant-carryingchannels 522, each of which may comprise a microchannel structure, suchas described above. Note that five coolant-carrying microchannels aredepicted in FIG. 5B, by way of example only. More or lesscoolant-carrying channels may be defined within the heat sink base, asdesired. Heat from the electronic component is rejected to coolantwithin the coolant-carrying channels in the heat sink base. Two-phasecooling of the heat-generating electronic component is achieved by atleast partial vaporization of the coolant (i.e., working fluid) withinthe one or more coolant-carrying channels of the heat sink.

As illustrated in FIGS. 5A & 5C, various regions of the coolant-carryingchannels are capped by at least one vapor-permeable region 524 ofmembrane structure 523. As illustrated in FIGS. 5A & 5D, disposed overthese regions are vapor transport channels 525 formed in heat sink cap526. Thus, localized venting of vapor directly from the coolant-carryingchannels, across the vapor-permeable membrane into the vapor transportchannels is provided within the heat sink. In one embodiment, membrane523 is modified to include, in addition to at least one vapor-permeableregion 524, at least one vapor-impermeable region 528. In oneembodiment, the at least one vapor-impermeable region 528 comprises aplurality of parallel-extending digits that are interdigitated with aplurality of vapor-permeable areas of the at least one vapor-permeableregion 524, as illustrated in FIG. 5C. The vapor-impermeable digitsextend substantially transverse to the coolant-carrying channels 522.

In the embodiment depicted, at least one orifice 550 is provided in eachof the vapor-impermeable digits where extending over a respectivecoolant-carrying channel. Coolant is introduced into thecoolant-carrying channels through orifices 550 via liquid coolantdelivery channels 527 in fluid communication with a liquid coolant inlet530 of heat sink 520. Coolant exhaust is discharged via coolant exhaustchannels 529 extending through an opening in membrane 523 into heat sinkcap 526. Coolant exhaust channels 529 are in fluid communication with acoolant exhaust outlet port 532 of heat sink 520. In this embodiment,the vapor transfer channel 525 vent within the heat sink into coolantexhaust channel 529, as illustrated in the plan view of FIG. 5D. Notethat in this embodiment, the orifices 550 in the vapor-impermeabledigits of the membrane are jet orifices, which provide jet impingementof coolant into the respective coolant-carrying channels of the heatsink. Note also that, in this embodiment, a single liquid coolant inletport and a single coolant exhaust outlet port are provided in the heatsink.

As illustrated in FIGS. 5B & 5D, heat sink base 521 and heat sink cap526 are configured to accommodate an O-ring 540 to seal coolant withinthe heat sink. Coolant and vapor are additionally sealed within the heatsink by vapor-impermeable region 528, which is provided to extend aroundthe perimeter of the membrane, that is, where held by the heat sink baseand heat sink cap as illustrated in FIG. 5A.

In one embodiment, the heat sink base and heat sink cap are fabricatedof a metal material, such as copper, the coolant comprises water, andthe membrane is a porous hydrophobic membrane, such as a vapor-permeablePTFE or polypropylene material, such as the membranes available, forexample, from Sterlitech Corp., of Kent, Wash., USA, or SumitomoElectric Interconnect Products, Inc., of San Marcos, Calif., USA.

FIG. 6 depicts one embodiment of a rack-level cooling apparatuscomprising multiple heat sink structures, such as depicted in FIGS.5A-5D. In this embodiment, two heat sink structures 520 are illustratedwithin an electronic subsystem 610, such as a node of an electronicsrack 600. The cooling apparatus includes a modular cooling unit 620,such as described above. Modular cooling unit 620 includes aliquid-to-liquid heat exchanger 621 and a reservoir with an associatedpump 622 for providing cooled liquid coolant via a coolant supplymanifold 623 and node-level supply lines 624, 625 to the coolant inletports of the respective heat sinks 520. In the embodiment of FIGS.5A-5D, the vented vapor is combined within the heat sink with thecoolant exhaust so that a single coolant exhaust line 626, 627 extendsfrom each heat sink 520. These coolant exhaust lines 626, 627 arecoupled in fluid communication (in this embodiment) at the node levelinto a single node-level coolant exhaust line 628, which is coupled influid communication with a phase separation manifold 630 of the rackunit. Phase separation manifold 630 comprises a buoyancy-driven phaseseparator, with the coolant exhaust comprising (in one mode ofoperation) both vapor and liquid. Vapor within the manifold rises to avapor condenser 640 disposed in an upper region of the electronics rack600. In one embodiment, the condenser is liquid-cooled 641, for example,via a facility coolant. The resultant condensate is returned via acondensate return line 642 to the liquid coolant return line 631coupling the phase separation manifold 630 to the modular cooling unit620, to repeat the process.

FIG. 7 depicts a more detailed embodiment of the cooling apparatus andelectronics rack of FIG. 6. In this embodiment, heat exchanger 621 ofmodular cooling unit 620 is shown to comprise a liquid-to-liquid heatexchanger, with a facility coolant loop 700 providing facility coolantto the liquid-to-liquid heat exchanger, as well as to vapor-condenser640. In the embodiment of FIG. 7, the phase separation manifold 630 isshown to comprise an elongate, vertically-oriented structure, such as along tube with a relatively large internal diameter. Multiple nodes 610are also illustrated in FIG. 7, with each node receiving liquid coolantvia coolant supply manifold 623, and rejecting (in one embodiment)two-phase coolant exhaust via node-level coolant exhaust line 628 tophase separation manifold 630.

Referring collectively to FIGS. 5A-7, operationally, at low heat fluxes,coolant impinges on the coolant-carrying channel surfaces of the heatsink base and flows down the coolant-carrying channels as a single-phaseliquid to the coolant exhaust plenum at either end of the channels. Theliquid-impermeable nature of the vapor-permeable membrane stops theliquid from leaking from the coolant-carrying channels through the poresof the membrane into the vapor transport channels in the heat sink cap.The liquid impingement has a higher heat transfer coefficient, and therelatively shorter flow lengths facilitate reducing flow pressure drop,and may maintain better temperature uniformity compared with coolantdelivery parallel to the heated surface. The liquid flows to theexternal cooling apparatus (as shown in FIGS. 6 & 7), where it dropsdown the phase separation manifold to the modular cooling unit. Withinthe modular cooling unit, the heated coolant is cooled by the heatexchanger, with heat being rejected to the facility coolant passingthrough the heat exchanger. The cooled liquid coolant is then pumpedback to the nodes of the electronics rack, and in particular, to flowthrough the heat sinks, in a manner such as described above.

At higher heat fluxes, a portion of the impinging coolant vaporizeswithin the coolant-carrying channels, with a liquid and vapor mixtureflowing down the length of the channels. However, the vapor phase mayalso egress through the vapor-permeable region(s) of the membrane intothe vapor transport channels of the heat sink cap, leaving a relativelyliquid-rich coolant exhaust flowing in the coolant channels. This localremoval of the vapor helps maintain a high heat transfer coefficient,reduces the pressure drop, and reduces dryout within the heat sink. Theseparated vapor can then be reintroduced into the coolant exhaust (e.g.,a two-phase exhaust mixture) exiting from the edges of the heat sinkbase, through the large openings in the membrane, to the heat sink cap(see FIGS. 5A-5D). The reintroduction of the vapor in the heat sink capdoes not significantly add to the pressure drop, due to the largerlength scales of the channels in the heat sink cap. Doing so alsosimplifies plumbing external to the heat sink, as shown schematically inFIG. 6. The two-phase coolant effluent then flows to the separationmanifold of the electronics rack, where the liquid drops down to themodular cooling unit, and the vapor rises to the vapor condenserdisposed in the upper region of the electronics rack. Within the vaporcondenser, the vapor is condensed, for example, with the assistance offacility chilled water passing through the vapor condenser. Thecondensed liquid then flows down the rack to be merged with the liquiddrip from the phase separation manifold, and enters the modular coolingunit to be chilled and pumped back to the nodes to repeat the process.

FIGS. 8A-8D depict an alternate embodiment of a cooled electronicsstructure, generally denoted 800, in accordance with one or more aspectsof the present invention. Cooled electronic structure 800 is similar tocooled electronic structure 500 of FIGS. 5A-5D, except the layers thatmake up heat sink 801 of FIGS. 8A-8D are modified from the layers thatmake up heat sink 520 (see FIG. 5A).

Specifically, as shown in FIG. 8A, cooled electronic structure 800includes, in this embodiment, electronic component 510, such as anelectronic module, mounted to printed circuit board 501, with anassociated back plate 502. Heat sink 801 is mechanically coupled viasecuring mechanisms 505 to back plate 502 of printed circuit board 501,which provide compressive loading of heat sink 801 to electroniccomponent 510. Electronic component 510 includes, in this embodiment,integrated circuit chip 511 connected to chip carrier or substrate 513via a first plurality of solder ball connections 512. Substrate 513 iselectrically connected to printed circuit board 501 via a secondplurality of solder ball connections 514. A thermally conductive cap 516is interfaced to integrated circuit chip 511 via first thermal interfacematerial 515, and to heat sink 801 via second interface material 517,which may be the same or different interface materials.

Heat sink 801 is again a multilayer heat sink with a heat sink base 810,a membrane structure 820, and a heat sink cap 830, which arerespectively depicted in cross-sectional plan view in FIGS. 8B-8D.Referring collectively to FIGS. 8A-8D, heat sink base 810 comprises oneor more coolant-carrying channels 812, each of which may comprise amicrochannel structure, such as described above. In operation, heat fromthe electronic component is rejected to coolant within thecoolant-carrying channels in the heat sink base 810, causing boiling ofthe coolant.

As illustrated in FIGS. 8A & 8C, various regions of the coolant-carryingchannels are capped by at least one vapor-permeable region 821 ofmembrane 820. As illustrated in FIGS. 8A & 8D, disposed over theseregions are vapor transport channels 831 formed in heat sink cap 830.Thus, localized venting of vapor 833 directly from the coolant-carryingchannels, across the vapor-permeable membrane into the vapor transportchannels is provided within the heat sink. In one embodiment, membrane820 is modified to include, in addition to the at least onevapor-permeable region 821, at least one vapor-impermeable region 822.The at least one vapor-impermeable region 822 comprises a plurality ofparallel-extending digits that are interdigitated with a plurality ofvapor-permeable areas of the at least one vapor-permeable region 821, asillustrated in FIG. 8C. The vapor-impermeable digits extendsubstantially transverse to the coolant-carrying channels 812.

In the embodiment depicted, at least one orifice 860 is provided in eachof the vapor-impermeable digits where extending over a respectivecoolant-carrying channel. Coolant 834 is introduced into thecoolant-carrying channels through orifices 860 via liquid coolantdelivery channels 832, which as illustrated in FIG. 8D, areinterdigitated with the vapor transport channels 831 within the heatsink cap 830. Liquid coolant delivery channels 832 are in fluidcommunication with a liquid coolant inlet port 840 of heat sink 801.Coolant exhaust is discharged via coolant exhaust channels 814 through acoolant exhaust outlet port 842. In this embodiment, the vapor transportchannels 831 vent vapor from the heat sink through a vapor outlet port841, as illustrated in FIG. 8D.

As with the cooled electronic structure embodiment of FIGS. 5A-5D, heatsink base 810 and heat sink cap 830 are configured to accommodate, inthis embodiment, an O-ring 850 to seal coolant and vapor within the heatsink. Coolant and vapor are additionally sealed within the heat sink bythe vapor-impermeable region 822 defined around the perimeter of themembrane 820, that is, where held by the heat sink base and heat sinkcap, as illustrated in FIG. 8A.

In one embodiment, the heat sink base and heat sink cap are fabricatedof a metal material, such as copper, the coolant comprises water, andthe membrane is a vapor-permeable, liquid-impermeable membrane (exceptfor the jet orifices), such as a vapor-permeable PTFE or polypropylenematerial.

FIG. 9 depicts one embodiment of a rack-level cooling apparatuscomprising multiple heat sink structures, such as depicted in FIGS.8A-8D. In this embodiment, two heat sink structures 801 are illustratedwithin an electronic subsystem 910, such as a node of an electronicsrack 900. The cooling apparatus includes a modular cooling unit 620,such as described above. Modular cooling unit 620 includes aliquid-to-liquid heat exchanger 621 and a reservoir with an associatedpump 622 for providing cooled liquid coolant via a coolant supplymanifold 623 and node-level supply lines 624, 625 to the coolant inletports of the respective heat sinks 801. In the embodiment of FIGS.8A-8D, the vented vapor and the coolant exhaust are dischargedseparately via, for example, coolant exhaust lines 911, 912 extendingfrom each heat sink 801. In the embodiment depicted, these exhaust linesare merged within the electronic subsystem or node 910 into a single,two-phase coolant outlet line 913, which is coupled in fluidcommunication with phase separation manifold 630. Phase separationmanifold 630 comprises a buoyancy-driven phase separator, with thecoolant exhaust comprising (in one mode of operation) both vapor andliquid. Vapor within the manifold rises to vapor condenser 640 disposedin the upper region of electronics rack 900. In one embodiment, thecondenser is liquid-cooled 641, for example, via facility coolant. Theresultant condensate is returned to the modular cooling unit 620 torepeat the process.

One advantage of the heat sink design of FIGS. 8A-9 over the heat sinkdesign of FIGS. 5A-7 is that the vapor reintroduction into the coolantstream is at the node level, which reduces the risk of vapor channelflooding by the coolant exhaust.

FIG. 10 depicts another embodiment of a rack-level cooling apparatuscomprising multiple heat sink structures, such as depicted in FIGS.8A-8D. In this embodiment, two heat sink structures 801 are againillustrated within an electronic subsystem 1010, such as a node of anelectronics rack 1000. The cooling apparatus includes a modular coolingunit 620, such as described above. Modular cooling unit 620 includes aliquid-to-liquid heat exchanger 621 and a reservoir with an associatedpump 622 for providing cooled liquid coolant via a coolant supplymanifold 623 and node-level supply lines 624, 625 to the coolant inletports of the respective heat sinks 801. In the embodiment of FIG. 10,the vapor outlet ports of the heat sinks are coupled to vapor vent lines1011, which are connected in fluid communication at the node level intoa single vapor vent outlet line 1012, which is also connected in fluidcommunication with phase separation manifold 630. Similarly, the coolantexhaust outlet ports of the heat sinks 801 are connected to respectivecoolant exhaust lines 1013, which are merged within the node into asingle coolant exhaust outlet line 1014 that is coupled in fluidcommunication with phase separation manifold 630. As noted above, phaseseparation manifold 630 is a buoyancy-driven phase separator, with thecoolant exhaust comprising (in one mode of operation) both vapor andliquid. Vapor within the manifold rises to the vapor condenser 640disposed in the upper region of electronics rack 1000. In oneembodiment, the condenser is liquid-cooled 641, for example, viafacility coolant. The resultant condensate is returned to the modularcooling unit 620 to repeat the process.

FIG. 11 depicts a more detailed embodiment of the cooling apparatus andelectronics rack of FIG. 10. In this embodiment, heat exchanger 621 ofmodular cooling unit 620 is shown to comprise a liquid-to-liquid heatexchanger, with the facility coolant loop 700 providing facility coolantto the liquid-to-liquid heat exchanger, as well as to the vaporcondenser 640. In the embodiment of FIG. 11, the phase separationmanifold 630 is shown to comprise an elongate, vertically-orientedstructure, such as a long tube with a relatively large internaldiameter. Multiple nodes 1010 are illustrated in FIG. 11, with each nodereceiving liquid coolant via coolant supply manifold 623, and rejectingvapor via vapor outlet line 1012, and coolant via coolant exhaust outletline 1014 to phase separation manifold 630. Vapor within the manifoldrises to vapor condenser 640 disposed in an upper region of electronicsrack 1000. In one embodiment, the condenser is liquid-cooled, forexample, via facility coolant flowing through facility coolant loop 700.The resultant condensate is returned via a condensate return line 642 tothe liquid coolant return line 631 coupling phase separation manifold630 to modular cooling unit 620, to repeat the process.

FIGS. 12A-12C depict another embodiment of a cooled electronicstructure, in accordance with one or more aspects of the presentinvention. The cooled electronic structure of FIGS. 12A-12C is similarto cooled electronic structure 500 of FIGS. 5A-5D, except that thesingle-layer membrane 523 of FIGS. 5A-5D is replaced by a multilayerstructure comprising (in one embodiment) a vapor-permeable membrane 1210disposed between two masking plates 1220, as illustrated in FIG. 12A.

Specifically, as shown in FIG. 12A, the cooled electronic structureincludes, in this embodiment, electronic component 510, such as anelectronic module, mounted to a printed circuit board 501, with anassociated back plate 502. Heat sink 1200 is mechanically coupled viasecuring mechanisms 505 to back plate 502 of printed circuit board 501,which provide compressive loading of heat sink 1200 to electroniccomponent 510. Electronic component 510 includes, in this embodiment,integrated circuit chip 511 connected to chip carrier or substrate 513via a first plurality of solder ball connections 512. Substrate 513 iselectrically connected to printed circuit board 501 via a secondplurality of solder ball connections 514. A thermally conductive cap 516is interfaced to integrated circuit chip 511 via first thermal interfacematerial 515, and to the heat sink 1200 via second thermal interfacematerial 517, which may be the same or different interface materials.

Heat sink 1200 is again a multilayer heat sink, with a heat sink base521, a multilayer membrane structure comprising masking plates 1220, andvapor-permeable membrane 1210, and a heat sink cap 526. By way ofexample, embodiments of masking plate 1220 and vapor-permeable membrane1210 are respectively depicted in cross-sectional plan view in FIGS. 12B& 12C.

Referring collectively to FIGS. 12A-12C, heat sink base 521 comprisesone or more coolant-carrying channels 522, each of which may comprise amicrochannel structure, such as described above. In operation, heat fromthe electronic component is rejected to coolant within thecoolant-carrying channels in the heat sink base 521, causing (in onemode) boiling of the coolant.

As illustrated in FIGS. 12A-12C, various regions of the coolant-carryingchannels 522 are capped by vapor-permeable membrane 1210, which ispositioned between masking plates 1220 and exposed to thecoolant-carrying channels via open regions 1222 in masking plates 1220.These exposed regions of vapor-permeable membrane 1210 align to vaportransfer channels 525, which vent vapor egressing from thecoolant-carrying channels, as explained above.

Jet orifices or nozzles are defined in the multi-layer membranestructure via aligned through-holes 1221 in masking plates 1220, andthrough-holes 1211 in vapor-permeable membrane 1210. As explained above,these jet orifices inject coolant from liquid coolant delivery channels527 into the coolant-carrying channels 522 in heat sink base 521.

In one embodiment, masking plates 1220 comprise metal masking plates,which may be epoxied, soldered or press-fitted to heat sink base 521 andheat sink cap 526. Additionally, masking plates 1220 may be epoxied tothe vapor-permeable membrane 1210 for better sealing. Note also that theopen regions 1222 in the masking plate 1220 exposed to thecoolant-carrying channels 522 operate as vapor traps, where vaporcollects between the channels and the membrane. This further facilitatesegress of the vapor across the membrane into the vapor transportchannels 525. Note further, that in the depicted multilayer membranestructure embodiment, the vapor-permeable membrane of FIGS. 12A & 12Cneed not have a vapor-impermeable region, such as in the embodiments ofFIGS. 5A-11, described above. Note also that other multilayer membranestructure embodiments may alternatively be employed with a heat sinkstructure as described herein. For example, a single masking plate couldbe employed with the vapor-permeable membrane, if desired.

Those skilled in the art will note from the above discussion that theheat sink structures described herein include a heat sink base whichcomprises one or more coolant-carrying channels. In one embodiment,these coolant-carrying channels have sub-millimeter hydraulic diameters,and also are referred to herein as “microchannels”. Such small channelshelp increase the surface area, as well as the single-phase heattransfer coefficient of the coolant within the channels. The channelscan be made via chemical etching or mechanical methods, such as skivingor end-milling. In one embodiment, the heat sink is fabricated ofcopper, due to its high heat transfer coefficient and relatively simplemachineability. However, other materials, such as aluminum and siliconare also suitable, though may have disadvantages in terms of thermalconductivity, fragility and machineability.

The second layer of the heat sink comprises a vapor-permeable membrane,such as a porous, hydrophobic membrane, in the case where the coolantcomprises water. Examples of micro/nano-porous, natively hydrophobicmembranes include polypropylene, PTFE, and nylon. Natively hydrophilicmaterials, such as porous glass, porous silicon, porous aluminum andporous organic materials could also be used, but require a liquid-phobiccoating to prevent liquid from leaking into the vapor channels. Theporous membrane is prepared such that the regions with the nozzles ororifices, as well as the edges of the membrane, are hardened andnon-porous to provide better nozzle definition as well as edge sealing.The membrane can be patterned using a variety of techniques, such as hotpress (wherein a heated master is pressed onto the porous membrane toplastically deform it and close the pores in the desired regions),laminating with a non-porous material (one example of which is laminatedporous PTFE, where the laminate is made of non-porous polypropylene), orepoxy/photoresist infiltration (where epoxy could be used to selectivelyclose the pores and provide additional mechanical stiffness in desiredregions).

In an alternate embodiment, the second layer of the heat sink mightcomprise a multilayer membrane structure, for example, such as depictedin FIGS. 12A-12C, and described above. In such a multilayer structure,the membrane may be a vapor-permeable membrane, for instance, a porous,hydrophobic membrane, in the case where the coolant comprises water.Additionally, the masking plate may be fabricated of variousvapor-impermeable materials, with metal being one example.

The third layer of the heat sink, that is, the heat sink cap, comprisesrelatively larger liquid and vapor channels which help distribute thefluid from and to the inlet and outlet ports of the heat sink. In orderto minimize the pressure drop in these channels, the hydraulic diameteris maintained relatively large. A large hydraulic diameter also reducesthe pressure drop when the vented vapor is reintroduced to the coolanteffluent (which may be a two-phase effluent) at the heat sink level. Theheat sink cap can be made of copper or aluminum or any other materialwith a similar coefficient of thermal expansion (CTE) as that of theheat sink base to avoid excessive thermal stresses developing.

The coolant (or working fluid) should be compatible with the selectedmembrane, thus requiring specific fluid/membrane combinations. Examples,of coolants (or working fluids) include: water at sub-ambient pressures,dielectric fluids at atmospheric pressure, and refrigerants at higherpressures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include” (and any formof include, such as “includes” and “including”), and “contain” (and anyform contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises”, “has”,“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises”, “has”, “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.

What is claimed is:
 1. A method of facilitating extraction of heat froma heat-generating electronic component, the method comprising: providinga heat sink comprising: a thermally conductive structure comprising atleast one coolant-carrying channel; and a membrane structure associatedwith the at least one coolant-carrying channel, the membrane structurecomprising at least one vapor-permeable region, at least a portion ofthe at least one vapor-permeable region overlying a portion of the atleast one coolant-carrying channel and facilitating removal of vaporfrom the at least one coolant-carrying channel, and the membranestructure further comprising at least one orifice coupled to injectcoolant onto at least one surface of the at least one coolant-carryingchannel intermediate ends of the at least one coolant-carrying channel;at least one vapor transport channel in fluid communication with the atleast one vapor-permeable region of the membrane structure, the at leastone vapor transport channel facilitating exhausting of vapor egressingacross the at least one vapor-permeable region of the membrane structurefrom the at least one coolant-carrying channel; at least one coolantexhaust channel in fluid communication with the at least onecoolant-carrying channel and facilitating exhausting of coolant from theat least one coolant-carrying channel; and a coolant exhaust outlet portcoupled in fluid communication with the at least one coolant exhaustchannel, and a vapor outlet port coupled in fluid communication with theat least one vapor transport channel for separately discharging coolantexhaust from the at least one coolant exhaust channel and vapor from theat least one vapor transport channel; and coupling the heat sink to theat least one heat-generating electronic component so that heat generatedby the at least one heat-generating electronic component is dissipatedto coolant within the at least one coolant-carrying channel of the heatsink, wherein vapor generated within the at least one coolant-carryingchannel can exhaust from the at least one coolant-carrying channelacross the at least one vapor-permeable region of the membranestructure.
 2. The method of claim 1, wherein the membrane structurecomprises a liquid-impermeable membrane, and the at least onecoolant-carrying channel comprises at least one coolant-carryingmicrochannel having a characteristic dimension less than 1.0 mm.
 3. Themethod of claim 1, wherein the membrane structure further comprises atleast one vapor-impermeable region, the at least one vapor-impermeableregion comprising the at least one orifice coupled to inject coolantinto the at least one coolant-carrying channel.
 4. The method of claim1, wherein the membrane structure further comprises at least onevapor-impermeable region, the at least one vapor-impermeable regioncomprising multiple vapor-impermeable digits, each vapor-impermeabledigit comprising at least one orifice coupled to inject coolant into theat least one coolant-carrying channel, the multiple vapor-impermeabledigits being interdigitated with multiple vapor-permeable areas of theat least one vapor-permeable region.
 5. The method of claim 4, whereinthe thermally conductive structure comprises multiple coolant-carryingchannels arranged substantially parallel to each other, and wherein themultiple vapor-impermeable digits extend transverse to the multiplecoolant-carrying channels, and each vapor-impermeable digit of themembrane comprises multiple orifices, each orifice of the multipleorifices being coupled to inject coolant into a respectivecoolant-carrying channel of the multiple coolant-carrying channels. 6.The method of claim 1, wherein coolant within the at least onecoolant-carrying channel comprises water, and wherein the membranestructure comprises a hydrophobic membrane.